The present invention relates to current conversion circuits, and more particularly to a current conversion circuit for converting an output voltage of a rectifier circuit into a proportional current.
A conventional device of this type is shown in FIG. 1. An input signal is applied to an input terminal 1. An operational amplifier OP1, resistors R.sub.1, R.sub.2 and R.sub.3, and diodes D.sub.1 and D.sub.2 form a conventional half-wave rectifier circuit. The junction point of the resistor R.sub.2 and the cathode of the diode D.sub.2 is connected to one terminal of a resistor R.sub.4, the other terminal of which is connected to the noninverting input terminal of an operational amplifier OP2, one terminal of a resistor R.sub.5 and one terminal of a capacitor C.sub.1. The other terminals of the resistor R.sub.5 and the capacitor C.sub.1 are grounded.
The output terminal of the operational amplifier OP2 is connected to the base of a transistor Q.sub.1, the emitter of which is connected to the inverting input terminal of the operational amplifier OP2 and one terminal of a resistor R.sub.6, the other terminal of which is grounded. The operational amplifier OP2, the transistor Q.sub.1 and the resistor R.sub.6 form a so-called current conversion circuit.
The operation of the device will be described with reference to FIGS. 2A to 2D which illustrate signal waveforms at various circuit points in the device. When an input v.sub.1 as shown in FIG. 2A is applied to the input terminal 1, a waveform v.sub.2 as shown in FIG. 2B, which is obtained by subjecting the input v.sub.1 to inversion amplification and to half-wave rectification, is provided at the cathode of the diode D.sub.2. Accordingly, at the junction point of the resistors R.sub.4 and R.sub.5, the capacitor C.sub.1 and the noninverting input terminal of the OP2, a signal v.sub.3 having a waveform as shown in FIG. 2C is obtained by a time constant circuit composed of the resistors R.sub.4 and R.sub.5. In this case, the resistance of the resistor R.sub.4 is much smaller than that of the resistor R.sub.5 (R.sub.4 &lt;&lt;R.sub.5).
The operational amplifier OP2 and the transistor Q.sub.1 form a voltage follower. Therefore, the waveform of the emitter output v.sub.4 of the transistor Q.sub.1 is similar to that of the signal v.sub.3. Accordingly, if the base current of the transistor Q.sub.1 is disregarded, then the waveform of the collector current (i.e., the output current i.sub.0) as shown in FIG. 2D is similar to that shown in FIG. 2C. This output current is used as a control current for gain control, for instance.
The conventional current conversion circuit, designed as described above, suffers from drawbacks in that it is intricate and needs a number of components, and accordingly it is high in manufacturing cost.